Multiple access radio systems provide communication services for fixed remote user terminals and/or mobile user terminals. Examples of multiple access radio systems include land mobile radio networks, cellular mobile radio networks, and wideband radio networks between fixed subscribers and one or more central nodes, which may use a multibeam antenna for increasing system capacity and improving communications quality. The reverse link or uplink in a multiple access radio system is a communications link between a fixed remote or mobile user terminal and a central node, which can be located at either a fixed location on the Earth in a terrestrial radio system or as part of an orbiting satellite in a satellite radio system.
Digital radio systems transmit and receive digital message information, e.g., computer or Internet data. Alternatively, digital radio systems accept analog message information, e.g., voice or video data, and convert this analog information to a digital format during transmission and reception. Accordingly, a fixed remote or mobile user terminal transmits message information in a digital format using an uplink to a central node, where a multibeam antenna and associated receiver process received signals to extract user message information. In some satellite radio systems, the receiver processing is divided between a satellite repeater and a ground-based station processor.
User terminals within the same beam coverage region generally avoid mutual interference through the use of some form of multiple access scheme. Conventional multiple access radio services use Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), or some combination thereof. Generally, FDMA separates users into different frequency subbands; TDMA separates users into different time intervals or slots; and, CDMA separates users by assigning different signature waveforms or spreading codes to each user. These CDMA spreading codes can be either orthogonal, i.e., there is no interference between synchronized users, or quasi-orthogonal, i.e., there is some small interference between users. FDMA and TDMA are orthogonal multiple access (OMA) schemes because with ideal frequency filters and synchronization there is no mutual interference. Another example of an OMA system is CDMA with orthogonal spreading codes. Quasi-Orthogonal Multiple Access (QOMA) systems include CDMA with quasi-orthogonal spreading codes and FDMA/TDMA with randomized frequency hopping.
In both orthogonal and quasi-orthogonal multiple access systems, the multiple access channels are usually assigned by a centralized controller which may make assignments for a single central node beam coverage region or the assignments may cover the beam coverage regions of multiple central nodes. The assignments to the user terminals are normally transmitted in time division with downlink message information. After synchronization, user terminals can extract the channel assignment data from the downlink messages.
For an isolated beam, an OMA scheme generally provides a larger system capacity than a QOMA scheme. However, when other beams are taken into account, practical systems often use QOMA schemes for reducing interference between users to acceptable levels.
Interference between a user on one beam and users on other beams is normally reduced by user/beam cross-channel attenuation. However, in OMA radio systems, such cross-channel attenuation usually does not reduce interference enough to allow the reuse of the same orthogonal waveform or channel in adjacent beams. Instead, channel management is typically required for determining when a multiple access channel can be reused in another beam depending on an allowable threshold of the user/beam cross-channel attenuation. This leads to a reuse factor that is less than 1. The reuse factor of a multiple access channel is defined as the number of user terminal assignments in different beam coverage regions divided by the total number of beam coverage regions. Because the capacity of a multiple access system is proportional to the average value of the reuse factor with respect to all the multiple access channels, it is desirable to make the reuse factor for each multiple access channel as large as possible subject to interference constraints.
Practical limitations on multibeam antennas typically cause the reuse factor in cellular OMA systems to vary between ⅓ and 1/12.
In contrast, in a QOMA radio system, e.g., the uplink of a CDMA radio system in the IS-95 standard, the reuse factor can be unity because the combination of user/beam cross-channel attenuation and spreading code interference protection is sufficient to keep mutual interference between users in different beams to adequately small levels. However, one drawback is that a QOMA radio system generally has a theoretical capacity that is less than that of an OMA radio system.
Conventional multiple access digital radio systems provide means for error-correction coding/decoding of message information, means for interleaving/deinterleaving the message information, and a transmission format for the message information that includes reference signal sub-blocks. The reference signal is generated at both the user terminal and the central node and used by the central node receiver for obtaining channel parameters to aid in demodulating a user signal. The insertion of a known reference signal in time multiplex with the transmitted message information is described in “An Adaptive Receiver for Digital Signaling through Channels with Intersymbol Interference”, J. G. Proakis and J. H. Miller. IEEE Transactions on Information Theory, vol. IT-15, No. 4, July 1969 and in U.S. Pat. No. 4,365,338. Error-correction coding adds redundancy to message information in a prescribed manner so that transmission errors may be corrected with a decoder at the receiver. The purpose of the interleaver/deinterleaver is to randomize these transmission errors at the decoder input so as to make the decoder more capable of correcting them.
Further, the message information conventionally undergoes quadrature transmission, wherein two carriers in phase quadrature to one another, e.g., cos ωct and sin ωct, are simultaneously transmitted using the same channel. Quadrature transmission is an example of a multisymbol signaling scheme, wherein pluralities of successive binary digits of user data are combined to form symbols to be transmitted. Such multisymbol signaling schemes are typically used to reduce the bandwidth required to transmit the user data. Quadrature amplitude modulation (QAM) is an example of a general multisymbol signaling scheme, wherein multilevel amplitude modulation is applied separately on each of the two quadrature carriers.
Some conventional digital radio systems use adaptive equalizers for combining multipath signals and reducing intersymbol interference. Adaptive equalizers have also been proposed for use with a multibeam receiver for reducing interference from other users.
MMSE Equalization of Interference on Fading Diversity Channels, Peter Monsen, IEEE Conference on Communications, Conference Record, Vol. 1, Denver, Colo., June 1981, pp. 12.2-1–12.2-5, describes an adaptive equalizer that combines multipath signals and reduces intersymbol and other user interference. The total interference is included in an error signal whose mean square value is minimized. Transmission of a time division multiplexed reference that is known at the receiver is also described.
U.S. Pat. Nos. 4,112,370 and 4,644,562 disclose the cancellation of interference in multibeam antennas as a generalization of the cancellation of interference in dual-polarized antennas.
U.S. Pat. No. 5,680,419 discloses adaptive sequence estimation techniques that can be used with a multibeam antenna for canceling interference. Adaptive Equalization and Interference Cancellation for Wireless Communication Systems, B. C. W. Lo and K. B. Letaief, IEEE Trans. Comm., vol. 47, no. 4, April 1999, pp. 538–545 discloses in a multiantenna application a maximum likelihood sequence estimation technique that uses a reference signal of the desired user in order to detect a user signal in the presence of intersymbol interference and other user interference. Although either an equalizer or a sequence estimator or a combination of both can be used for adaptive processing, the equalizer is generally preferred because it is not as complex as the sequence estimator.
Other relevant patent documents and publications include U.S. Pat. No. 5,838,742; Dynamic Channel Assignment in High-Capacity Mobile Communications Systems, D. C. Cox and D. O. Reudick, Bell System. Tech. Journal, vol. 51, pp. 1833–2857, July–August 1971; MMSE Equalization on Fading Diversity Channels, P. Monsen, IEEE Transactions on Communications, vol. COM-32, No. 1, pp. 5–12, January 1984; Linear Multiuser Detectors for Synchronous Code-Division Multiple Access Channels, R. Lupas and S. Verdu, IEEE Transactions on Information Theory, vol. IT-35, No. 1, pp. 123–136, January 1989; Decorrelating Decision-Feedback Multiuser Detector for Synchronous Code-Division Multiple Access Channels, A. Duel-Hallen, IEEE Transactions on Communications, vol. COM-41, No. 2, pp. 285–290, February 1993; A Family of Multiuser Decision Feedback Detectors for Asynchronous Code-Division Multiple Access Channels, A. Duel-Hallen, IEEE Transactions on Communications, vol. COM-43, Nos. 2,3,4, February–April 1995; Information-Theoretic Considerations for Symmetric, Cellular, Multiple Access Fading Channels-Part I, S. Shamai and A. D. Wyner, IEEE Transactions on Information Theory, vol. 43, No. 6, pp 1877–1894, November 1997; and, Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, EIA/TIA IS-95, 1992.
Although the techniques described above have been used for improving communications quality and increasing the capacity of multiple access communication systems, it has been recognized that the capacity of OMA systems is limited because its multiple access channels have reuse factors less than 1. It has also been recognized that the capacity of QOMA systems is limited because its theoretical capacity is less than that of a corresponding OMA system.